Microelectronic devices are fabricated on a semiconductor substrate as integrated circuits in which various conductive layers are interconnected with one another to permit electronic signals to propagate within the device. An example of such a device is a complementary metal-oxide-semiconductor (CMOS) field effect transistor (FET) or MOSFET.
Over the past decades, the MOSFET has continually been scaled down in size and modern integrated circuits are incorporating MOSFETs with channel lengths of less than 0.1 micron. The decrease in feature size has resulted in certain challenges because small MOSFETs exhibit higher leakage currents, and lower output resistance than larger devices. Still, smaller MOSFETs are desirable for several reasons. The main reason to make transistors smaller is to pack more and more devices in a given chip area, reducing the price per chip. Additionally, the reduction in transistor dimension can help increase the speed.
Because of small MOSFET geometries, the voltage that can be applied to the gate must be reduced to maintain reliability. To maintain performance, the threshold voltage of the MOSFET has to be reduced as well. As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available. Subthreshold leakage, which was ignored in the past, now can have a significant impact on device performance.
A gate electrode is part of an integrated circuit. For example, a CMOS transistor comprises a gate structure disposed between source and drain regions that are formed in the semiconductor substrate. The gate structure generally comprises a gate electrode and a gate dielectric. The gate electrode is disposed over the gate dielectric to control a flow of charge carriers in a channel region that is formed between drain and source regions beneath the gate dielectric. The gate dielectric typically comprises a thin material layer having a dielectric constant of about 4.0 or greater (for example, gate oxides such as silicon dioxide (SiO2), silicon oxynitride (SiON), and the like). As the gate length of silicon CMOS devices is scaled to less than 100 nm, new high dielectric constant (K) materials will likely replace silicon oxide. In addition, metal gates will likely replace polycrystalline silicon (polysilicon) gates. For example, in some CMOS transistors, the gate electrode may be formed from at least one of a metal (e.g., titanium (Ti), tantalum (Ta), tungsten (W), and the like) and metal-containing conductive compound (e.g., titanium nitride (TiN), tantalum nitride (TaN), tungsten nitride (WN), and the like). Replacement of polysilicon as a traditional material of the gate electrode with metals and metal-containing compounds reduces undesired voltage drops associated with the polysilicon depletion effect, as well as increases drive current performance and the operational speed of the CMOS transistor.
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 1.2 nm, the quantum mechanical phenomenon of electron tunneling occurs between the gate and channel, leading to increased power consumption.
High-k dielectrics that have a larger dielectric constant than silicon dioxide are being used to reduce the gate leakage, such as hafnium oxide (HfO2). Increasing the dielectric constant of the gate dielectric allows a thicker layer while maintaining a high capacitance. Generally, a higher dielectric thickness reduces the quantum tunneling current through the dielectric between the gate and the channel. However, the difference in conduction band energy between the semiconductor and the dielectric (and the corresponding difference in valence band energy) impacts leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics, the value is significantly lower, tending to increase the tunneling current, somewhat negating the advantage of higher dielectric constant.
As mentioned above, alternative materials have been proposed for use as gate dielectric materials. Although improvements to semiconductor gate electrodes have been made through the use of alternative gate metals and gate dielectric materials, further improvements are desired to improve the performance of integrated circuit devices, for example, to reduce leakage current density.
Thus, there is an ongoing need in the art for methods of rapidly and efficiently depositing dielectric films.